Organizations, such as schools, hospitals, and businesses, may maintain centrally located banks of laptops for checkout or use in classrooms, labs, and meetings. It is desirable to have the laptops fully charged when needed. In addition, it is desirable to provide a central location to charge laptops, such as during lunch breaks. However, charging a number of laptops simultaneously presents challenges. First, each laptop computer may have an internal or external power supply that may draw several amperes during use. Thus the total number of laptops that may be charged at one time is limited by the branch circuit being used (e.g., the upstream circuit breaker/fuse and supply conductors). Furthermore, the laptops may utilize power supplies characterized by an initial current inrush.
FIG. 1 illustrates a simplified switch mode power supply 100 that may be used to supply power to a laptop computer. The power supply 100 may receive power from an AC source, such as a branch circuit supplying 120 Vac (nominal), at terminals 105. A rectifier 110 generates a DC voltage that is applied to primary windings 116 of transformer 115. A controller 120, such as a pulse width modulator, switches transistor 125 on and off (generally at a higher frequency than the AC source), thereby allowing current to flow through the primary windings 116 of the transformer 115 and a voltage to be induced at secondary windings 117. The secondary windings 117 of the transformer 115 generally step down the AC voltage, and a diode 130 generates a DC voltage that is available at terminals 135. Feedback may be provided to the controller 120 via an opto-isolator 140. A capacitor 145 may be provided to smooth the DC voltage from the rectifier 110 and prevent high-frequency waveforms from being feedback onto the AC source. In addition, a capacitor 150 may be provided after diode 130.
The power supply 100 may have a large current inrush when first plugged into the AC source. The large inrush is mostly attributable to initially charging energy storing components, such as the capacitors 145 and 150 and the windings 116 and 117 of the transformer 115. The present inventor has recognized that attaching a large number of power supplies to the AC source at one time increases the initial inrush and may actually trip the branch circuit protection (e.g., the upstream 15 or 20 ampere circuit breaker or fuse). Further, the large inrush may weld together and/or cause excessive wear of the contacts of any switches located between the branch circuit protection and the power supplies. Thus the present inventor has recognized a need for improved devices and methods to power up a set of power supplies.